CN114342346A

CN114342346A - Correcting in-track errors in a linear printhead

Info

Publication number: CN114342346A
Application number: CN202080063086.6A
Authority: CN
Inventors: C-H·郭; B·B·米克斯; M·T·多伯廷
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 2019-09-09
Filing date: 2020-08-25
Publication date: 2022-04-12
Also published as: EP4029234A1; WO2021050257A1

Abstract

A method for correcting in-track position errors in a digital printing system having a linear printhead includes printing a test target including a plurality of alignment marks. A data processing system is used to automatically analyze the captured images of the printed test targets to determine a measured in-track position for each of the alignment marks. The measured in-track position for the alignment mark is compared to a reference position to determine a measured in-track position error. An in-track position correction function is determined in response to the measured in-track position error, wherein the in-track position correction function specifies an in-track position correction to be applied as a function of cross-track position. The corrected digital image is determined by resampling the input digital image in response to the in-orbit position correction function.

Description

Correcting in-track errors in a linear printhead

Technical Field

The present invention relates to the field of digital printing, and more particularly to a method for calibrating a printer including a linear printhead to compensate for in-track registration (registration) errors.

Background

Electrophotography (electrophotography) is a useful process for printing an image onto a receiver (or "imaging substrate") such as a sheet or sheet of paper or another planar medium (e.g., glass, fabric, metal, or other object) as will be described below. In this process, an electrostatic latent image is formed on the photoreceptor by uniformly charging the photoreceptor and then discharging selected regions of uniform charge to produce an electrostatic charge pattern corresponding to the desired image (i.e., the "latent image").

After the latent image is formed, the charged toner particles are brought into the vicinity of the photoreceptor and attracted to the latent image to develop the latent image into a toner image. Note that depending on the composition of the toner particles (e.g., transparent toner), the toner image may not be visible to the naked eye.

After the latent image is developed into a toner image on the photoreceptor, a suitable receptor is juxtaposed with the toner image. A suitable electric field is applied to transfer the toner particles of the toner image to a receiver to form a desired printed image on the receiver. The imaging process is typically repeated multiple times using a reusable photoreceptor.

The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (i.e., "fuse") the printed image to the receiver. Multiple printed images (e.g., separate images of different colors) can be overlaid on the receiver prior to fusing to form a multi-color printed image on the receiver.

In-track positional errors in digital printing systems with linear printheads can result in objectionable in-track alignment errors between color channels. Thus, there remains a need for a method to characterize and correct in-track position errors that can be achieved without the need for complex and expensive fixtures.

Disclosure of Invention

The present invention represents a method for correcting in-track position errors in a digital printing system having a linear printhead extending in a cross-track direction and including an array of light sources for exposing a photosensitive medium, the method comprising:

a) providing digital image data for a test target comprising a plurality of alignment marks positioned at predefined cross-track locations;

b) printing the test target using the digital printing system to provide a printed test target;

c) capturing an image of the printed test target using a digital image capture system;

d) automatically analyzing the captured images using a data processing system to determine a measured in-track position for each of the alignment marks;

e) comparing the measured in-track position for the alignment mark with a reference in-track position to determine a measured in-track position error;

f) determining an in-track position correction function in response to the measured in-track position error, wherein the in-track position correction function specifies an in-track position correction to be applied as a function of the cross-track position;

g) storing a representation of the in-track position correction function in a digital memory;

h) receiving digital image data for a digital image to be printed by a digital imaging system, wherein the digital image includes a plurality of image lines extending in a cross-track direction;

i) determining corrected image lines by resampling the digital image data in response to the stored representation of the in-orbit position correction function; and

j) the corrected image lines are printed using a digital printing system to provide a printed image with reduced in-track positional errors.

The present invention has the advantage of reducing in-track alignment errors in a digital printing system.

The present invention has the additional advantage of being able to determine an in-track position correction function using a simple process that includes printing and scanning a test target that includes a plurality of alignment marks.

The invention has the following additional advantages: the in-track position correction can be non-linear to account for local in-track alignment error characteristics.

Drawings

FIG. 1 is a front cross-section of an electrophotographic printer suitable for use with the various embodiments;

FIG. 2 is a front cross-section of one printing subsystem of the electrophotographic printer of FIG. 1;

FIG. 3 illustrates a conventional processing path for producing a printed image using a pre-processing system coupled to a print engine;

FIG. 4 illustrates a processing path including a print engine adapted to generate printed images from image data supplied by a variety of different pre-processing systems;

FIG. 5 shows additional detail for the resolution modification processor and the halftoning (halftone) processor of FIG. 4;

FIG. 6A shows an exemplary printhead including three light source dice each including 15 light source chips;

FIG. 6B shows an exemplary light source chip comprising a linear array of 384 LEDs;

FIG. 7 shows a flowchart of a process for determining a location correction function in accordance with an example embodiment;

FIG. 8 illustrates an exemplary test target including alignment marks useful for determining a position correction function;

9A-9B illustrate determining measured alignment mark positions from a combined image track;

FIG. 10A illustrates an exemplary cross-track position error function determined using the method of FIG. 6;

FIG. 10B illustrates an exemplary position correction function corresponding to the cross-track position error function of FIG. 10A;

FIG. 10C illustrates a position correction function representation corresponding to the position correction function of FIG. 10B;

FIG. 11 illustrates an improved processing path including a print engine adapted to generate a printed image incorporating cross-track position correction according to an exemplary embodiment;

FIG. 12 shows additional detail for the resolution/alignment processor and halftone processor of FIG. 11;

FIG. 13 illustrates a flowchart of a resampling operation for combining the resolution modification operation and the position correction operation of FIG. 12, according to an example embodiment;

FIG. 14 shows a flowchart of a process for determining an in-track position correction function, according to an example embodiment;

FIG. 15 illustrates an exemplary test target including in-track alignment marks useful for determining an in-track position correction function;

FIG. 16A illustrates an exemplary in-track position error function determined using the method of FIG. 14;

FIG. 16B illustrates an exemplary in-track position correction function corresponding to the in-track position error function of FIG. 16A;

FIG. 16C illustrates an exemplary in-track position correction function representation corresponding to the in-track position correction function of FIG. 16B;

FIG. 17 illustrates an improved processing path including a print engine adapted to generate a printed image incorporating in-track position corrections according to an exemplary embodiment;

FIG. 18 shows additional detail for the resolution/alignment processor and halftone processor of FIG. 17;

FIG. 19 illustrates an exemplary in-track position correction operation; and

FIG. 20 is a high level diagram showing components of a system for processing images in accordance with the present invention.

It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures.

Detailed Description

The invention includes combinations of the embodiments described herein. References to "a particular embodiment" or the like refer to features that are present in at least one embodiment of the invention. Separate references to "an embodiment" or "particular embodiments" or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive unless so indicated, or as is readily apparent to those of skill in the art. The use of the singular or plural in referring to "a method" or "methods" and the like is not limiting. It should be noted that the word "or" is used in this disclosure in a non-exclusive sense unless explicitly stated or required otherwise by context.

As used herein, the terms "parallel" and "perpendicular" have a tolerance of ± 10 °.

As used herein, a "sheet" is a discrete piece of media, such as receiver media for an electrophotographic printer (described below). The sheet has a length and a width. The sheet is folded along a fold axis (e.g., positioned at the center of the sheet in the length dimension and extending the entire width of the sheet). The folded sheet contains two "leaves", each leaf being the portion of the sheet on one side of the fold axis. Both sides of each leaf are called "pages". "face" refers to one side of the sheet, whether before or after folding.

As used herein, "toner particles" are particles of one or more materials that are transferred by an Electrophotographic (EP) printer to a receiver to produce a desired effect or structure (e.g., a printed image, texture, pattern, or coating) on the receiver. As is known in the art, toner particles can be milled or chemically prepared from larger solids (e.g., using an organic solvent to precipitate from a solution of pigment and dispersant). The toner particles can have a range of diameters (e.g., less than 8 μm, on the order of 10-15 μm, up to about 30 μm or more), where "diameter" preferably refers to a volume-weighted median diameter as determined by a device such as a Coulter Multisizer. In practicing the present invention, it is preferred to use larger toner particles (i.e., those having a diameter of at least 20 μm) in order to obtain a desired toner stack height that will enable the formation of macroscopic toner relief structures.

"toner" refers to a material or mixture of: contain toner particles and can be used to form images, patterns or coatings when deposited on imaging members comprising photoreceptors, photoconductors, or electrostatically charged or magnetic surfaces. The toner can be transferred from the imaging member to a receiver. Toners are also referred to in the art as marking particles, dry inks, or developers, but it is noted that "developers" are used differently herein, as described below. The toner can be a dry mixture of particles or a suspension of particles in a liquid toner base.

As already mentioned, the toner includes toner particles; the toner can also include other types of particles. The particles in the toner can be of various types and have various properties. Such properties can include absorption of incident electromagnetic radiation (e.g., particles containing colorants such as dyes or pigments), absorption of moisture or gases (e.g., desiccants or getters), inhibition of bacterial growth (e.g., biocides particularly useful in liquid toner systems), adhesion to receptors (e.g., adhesives), electrical conductivity or low magnetic resistance (e.g., metal particles), resistivity, texture, gloss, remanence, fluorescence, resistance to etchants, and other properties of additives known in the art.

In a single component or monocomponent development system, "developer" refers only to toner. In these systems, none, some, or all of the particles in the toner can be magnetic in nature. However, the developer in the one-component system does not include magnetic carrier particles. In a two, or multi-component development system, "developer" refers to a mixture comprising toner particles and magnetic carrier particles, which can be conductive or non-conductive. The toner particles can be magnetic or non-magnetic. The carrier particles can be larger than the toner particles (e.g., 15-20 μm or 20-300 μm in diameter). The magnetic field is used to move the developer in these systems by exerting a force on the magnetic carrier particles. The developer is moved by a magnetic field into proximity with the imaging member or transfer member, and the toner or toner particles in the developer are transferred from the developer to the member by the magnetic field, as will be described further below. By the action of the electric field, the magnetic carrier particles are not intentionally deposited on the component; only the toner is intentionally deposited. However, magnetic carrier particles and other particles in the toner or developer may be unintentionally transferred to the imaging member. The developer can include other additives known in the art, such as those listed above for the toner. The toner and carrier particles can be substantially spherical or non-spherical.

The electrophotographic process can be embodied in devices including printers, copiers, scanners and facsimile machines, as well as analog or digital devices, all of which are referred to herein as "printers". The various embodiments described herein may be used in electrophotographic printers, such as electrophotographic printers that employ toner developed on an electrophotographic receiver, as well as ionographic printers and copiers that do not rely on an electrophotographic receiver. Electrophotography and ionography are types of electrostatography (printing using electrostatic fields) which are a subset of electrography (printing using electric fields). The present invention can be practiced using any type of electrographic printing system, including electrophotographic printers and ionographic printers.

Digital reproduction printing systems ("printers") typically include a digital front end processor (DFE), a print engine (also referred to in the art as a "marking engine") for applying toner to a receiver, and one or more post-print finishing systems (e.g., a UV coating system, a glosser system, or a laminator system). The printer is capable of reproducing pleasing black and white or color images onto the receiver. The printer is also capable of producing a selected pattern of toner on the receiver that does not directly correspond to a visible image (e.g., surface texture).

In an embodiment of an electrophotographic modular printing machine (e.g., a NEXPRESS SX 3900 printer manufactured by Eastman Kodak corporation, rocchester, new york) that may be used in various embodiments, color toner print images are made in a plurality of color imaging modules arranged in tandem, and the print images are successively electrostatically transferred to a receiver attached to a transport web moving through the modules. Color toners include colorants (e.g., dyes or pigments) that absorb visible light of a particular wavelength. Commercial machines of this type typically employ an intermediate transfer member in the respective module in order to transfer the visible image from the photoreceptor and to transfer the printed image to the receiver. In other electrophotographic printers, each visible image is transferred directly to a receiver to form a corresponding printed image.

Electrophotographic printers are also known that have the ability to use an additional imaging module to also deposit clear toner. Providing a clear toner overcoat to color printing is desirable to provide features such as protecting the printing from fingerprints, reducing certain visual artifacts, or providing desirable texture or surface finish characteristics. Transparent toners use particles similar to the toner particles of a color development station, but without incorporating a color material (e.g., a dye or pigment) into the toner particles. However, transparent toner overcoats can increase cost and reduce the color gamut of printing; thus, it is desirable to provide an operator/user selection to determine whether the clear toner overcoat will be applied to the entire print. A uniform layer of transparent toner can be provided. Layers that vary inversely with the height of the toner stack can also be used to establish a level toner stack height. The respective color toners are deposited one on top of the other at respective locations on the receiver, and the height of the respective color toner stack is the sum of the toner heights of each respective color. The uniform stack height provides more even or uniform gloss to the print.

Fig. 1-2 are front cross-sections illustrating portions of a typical electrophotographic printer 100 that may be used in various embodiments. The printer 100 is adapted to produce an image on a receiver, such as a single color image (i.e., a monochrome image) or a multi-color image such as a CMYK or five color (five color) image. Multicolor images are also referred to as "multi-component" images. One embodiment involves printing using an electrophotographic print engine having a single color image generation or image printing station or module with five sets arranged in tandem, but enables more or less than five colors to be combined on a single receiver. Other electrophotographic writers or printer devices can also be included. The various components of the printer 100 are shown as rollers; other configurations are possible, including a belt.

Referring to fig. 1, a printer 100 is an electrophotographic printing apparatus having a number of electrophotographic image forming

printing subsystems

31, 32, 33, 34, 35 (which are also referred to as electrophotographic imaging subsystems) arranged in tandem. Each

printing subsystem

31, 32, 33, 34, 35 produces a single color toner image for transfer to a receiver 42 that moves sequentially through the module using a respective transfer subsystem 50 (only one indicated for clarity). In some embodiments, one or more of the

printing subsystems

31, 32, 33, 34, 35 can print a clear toner image that can be used to provide a protective overcoat or tactile image feature. Receiver 42 uses transport web 81 to transport from supply unit 40 into printer 100, supply unit 40 can include an active feed subsystem as is known in the art. In various embodiments, the visible image can be transferred directly from the imaging roll to the receiver, or sequentially from the imaging roll to one or more rolls or belts in the transfer subsystem 50 and then to the receiver 42. The receptor 42 is, for example, a selected section of mesh or a cut piece of planar receptor medium, such as paper or transparent film.

In the illustrated embodiment, each receiver 42 can have up to five single color toner images transferred thereon in registration during a single pass through the five

printing subsystems

31, 32, 33, 34, 35 to form a five-color image. As used herein, the term "five colors" means that in a printed image, various combinations of the five colors are combined to form other colors on the receiver at various locations on the receiver, and all five colors participate to form process colors in at least some of the subsets. That is, each of the five colors of toner can be combined with the toner of one or more of the other colors at a particular location on the receiver to form a color that is different from the color of the combined toner at that location. In an exemplary embodiment, the print subsystem 31 forms a black (K) print image, the print subsystem 32 forms a yellow (Y) print image, the print subsystem 33 forms a magenta (M) print image, and the print subsystem 34 forms a cyan (C) print image.

The print subsystem 35 is capable of forming a red, blue, green, or other fifth printed image that includes an image formed of a clear toner (e.g., one lacking a pigment). The four subtractive primary colors (cyan, magenta, yellow, and black) can be combined in various combinations of subsets thereof to form a representative color spectrum. The color gamut of the printer (i.e., the range of colors that can be produced by the printer) depends on the materials used and the process used to form the colors. Therefore, a fifth color can be added to improve the color gamut. In addition to being added to the color gamut, the fifth color can also be a specialty color toner or spot color, such as for making a proprietary logo or color that cannot be produced with CMYK-only colors (e.g., metallic, fluorescent, or pearlescent colors) or clear toners or tinted toners. Tinting toners absorb less light than they transmit, but do contain pigments or dyes that shift the hue of the light passing through them toward the hue of the tinting agent. For example, a blue-tinted toner coated on white paper will cause the white paper to appear bluish when viewed under white light, and will cause yellow printed under the blue-tinted toner to appear greenish under white light.

A receiver 42a is shown after passing through the printing subsystem 31. The printed image 38 on receiver 42a includes unfused toner particles. Following transfer of the respective printed images overlaid in registration from one of each of the

respective printing subsystems

31, 32, 33, 34, 35, the receiver 42a advances to a fuser module 60 (i.e., a fusing or fixing assembly) to fuse the printed images 38 to the receiver 42 a. Transport web 81 transports the printed image bearing receiver to fuser module 60, and transport web 81 typically applies heat and pressure to secure the toner particles to the respective receiver. The receivers are continuously disconnected from transport web 81 to allow them to be fed cleanly into fuser module 60. The transport web 81 is then reconditioned for reuse at the cleaning station 86 by cleaning and neutralizing the charge on the opposing surface of the transport web 81. A mechanical cleaning station (not shown) for scraping or vacuum-drawing toner off the transport web 81 can also be used independently or together with the cleaning station 86. Mechanical cleaning stations can be arranged along the transport web 81 before the cleaning station 86 or after the cleaning station 86 in the direction of rotation of the transport web 81.

In the illustrated embodiment, fuser module 60 includes a heated fusing roll 62 and an opposing pressure roll 64, fusing roll 62 and pressure roll 64 forming a fusing nip 66 therebetween. In an embodiment, fuser module 60 also includes a release fluid application sub-station 68 that applies a release fluid (e.g., silicone oil) to fusing roller 62. Alternatively, a toner containing wax can be used without applying a release fluid to the fuser roll 62. Other embodiments of the fuser, both contact and non-contact, can be employed. For example, solvent fixing uses a solvent to soften the toner particles so they bond with the receptor. Flash fusing uses short pulses of high frequency electromagnetic radiation (e.g., ultraviolet light) to melt the toner. Radiation fixing uses lower frequency electromagnetic radiation (e.g., infrared light) to melt the toner more slowly. Microwave fixing uses electromagnetic radiation in the microwave range to heat the receptor (primarily), thereby causing the toner particles to melt by thermal conduction, causing the toner to be fixed to the receptor.

The fused receiver (e.g., receiver 42b carrying fused image 39) is continuously transported from fuser module 60 along a path to output tray 69 or back to

printing subsystems

31, 32, 33, 34, 35 to form an image on the back of the receiver (i.e., to form a duplex print). The receiver 42b can also deliver to any suitable output accessory. For example, an auxiliary fuser or glosser assembly can provide a transparent toner overcoat. As is known in the art, printer 100 can also include multiple fuser modules 60 to support applications such as overprinting.

In various embodiments, receiver 42b passes through finisher 70 between fuser module 60 and output tray 69. The finisher 70 performs various sheet handling operations such as folding, binding, saddle stitching, collation, and binding.

Printer 100 includes a main printer device Logic and Control Unit (LCU) 99, LCU 99 receiving input signals from various sensors associated with printer 100 and sending control signals to various components of printer 100. LCU 99 can include a microprocessor incorporating suitable look-up tables and control software executable by LCU 99. LCU 99 can also include a Field Programmable Gate Array (FPGA), a Programmable Logic Device (PLD), a Programmable Logic Controller (PLC) with, for example, a program in ladder logic, a microcontroller, or other digital control system. LCU 99 can include memory for storing control software and data. In some embodiments, a sensor associated with fuser module 60 provides an appropriate signal to LCU 99. In response to the sensor signals, the LCU 99 issues command and control signals that adjust the heat or pressure within the fusing nip 66 and other operating parameters of the fuser module 60. This allows the printer 100 to print on receivers of various thicknesses and surface finishes, such as glossy or matte.

Fig. 2 shows additional details of printing subsystem 31, printing subsystem 31 representing printing subsystems 32, 33, 34, and 35 (fig. 1). The photoreceptor 206 of the imaging member 111 includes a photoconductive layer formed on a conductive substrate. The photoconductive layer is an insulator in the substantial absence of light so that charge is retained on its surface. Upon exposure to light, the charge is consumed. In various embodiments, the photoreceptor 206 is part of or disposed above the surface of the imaging member 111, which surface of the imaging member 111 can be a plate, drum, or belt. The photoreceptor can include a homogeneous layer of a single material, such as vitreous selenium, or a composite layer containing the photoconductor and another material. Photoreceptor 206 can also contain multiple layers.

The charging subsystem 210 applies a uniform electrostatic charge to the photoreceptor 206 of the imaging member 111. In an exemplary embodiment, the charging subsystem 210 includes a wire grid 213 having a selected voltage. Additional necessary components provided for control can be assembled around the various process elements of the respective printing subsystems. Meter 211 measures the uniform electrostatic charge provided by charging subsystem 210.

An exposure subsystem 220 is provided for selectively modulating uniform electrostatic charge on photoreceptor 206 imagewise by exposing photoreceptor 206 to electromagnetic radiation to form a latent electrostatic image. Uniformly charged photoreceptor 206 is typically exposed to actinic radiation provided by selectively activating specific light sources in an LED array or a laser device that outputs light directed onto photoreceptor 206. In embodiments using a laser device, a rotating polygon (not shown) is sometimes used to scan one or more laser beams across the photoreceptor in the fast scan direction. One pixel site is exposed at a time, and the intensity or duty cycle of the laser beam is varied at each point site. In embodiments using an LED array, which can include a plurality of LEDs arranged in a row immediately adjacent to one another, all of a row of spot locations on the photoreceptor can be selectively exposed simultaneously, and the intensity or duty cycle of each LED can be varied over a row exposure time to expose each pixel location in the row during the row exposure time.

As used herein, an "engine pixel" is the smallest addressable unit on photoreceptor 206 that exposure subsystem 220 (e.g., a laser or LED) can expose with a selected exposure that is different from the exposure of another engine pixel. The engine pixels can overlap (e.g., to improve addressability in the slow scan direction). Each engine pixel has a corresponding engine pixel location, and the exposure applied to the engine pixel location is described by the engine pixel level.

The exposure subsystem 220 can be a white writing or black writing system. In a white writing or "charged area development" system, the exposure dissipates charge on the areas of photoreceptor 206 to which toner should not adhere. The toner particles are charged to be attracted to the charge held on the photoreceptor 206. The exposed areas thus correspond to the white areas of the printed page. In a black writing or "discharged area development" system, the toner is charged to be attracted to a bias voltage that is applied to photoreceptor 206 and repelled by the charge on photoreceptor 206. Thus, the toner adheres to the area where the charge on the photoreceptor 206 has been consumed by exposure. The exposed areas thus correspond to the black areas of the printed page.

In the illustrated embodiment, a meter 212 is provided to measure the post-exposure surface potential within the patch area of the latent image formed from time to time in the non-image area on photoreceptor 206. Other instruments and components (not shown) can also be included.

Developing station 225 includes a toning shell 226, which toning shell 226 can be rotating or stationary, in order to apply toner of a selected color to the latent image on photoreceptor 206 to produce a developed image on photoreceptor 206 that corresponds to the color of toner deposited at the printing subsystem 31. The development stations 225 are electrically biased by suitable respective voltages, which can be supplied by a power supply (not shown), to develop the respective latent images. The developer is supplied to the toning shell 226 by a supply system (not shown), such as a supply roller, auger, or belt. Toner is transferred from the development station 225 to the photoreceptor 206 by electrostatic forces. These forces can include coulombic forces between the charged toner particles and the charged electrostatic latent image and lorentz forces on the charged toner particles due to the electric field generated by the bias voltage.

In some embodiments, the development station 225 employs a two-component developer that includes toner particles and magnetic carrier particles. As is known in the electrophotographic art, the exemplary development station 225 includes a magnetic core 227 to cause the magnetic carrier particles near the toning shell 226 to form a "magnetic brush. The magnetic core 227 can be stationary or rotating and can rotate at the same or a different speed and direction as the toning shell 226. The magnetic core 227 can be cylindrical or non-cylindrical, and can include a single magnet or multiple magnets or poles disposed around the circumference of the magnetic core 227. Alternatively, the magnetic core 227 can include an array of solenoids driven to provide magnetic fields of alternating directions. The magnetic core 227 preferably provides a magnetic field of varying magnitude and direction around the outer circumference of the toning shell 226. The development station 225 can also employ a one-component developer that includes a magnetic or non-magnetic toner in the absence of individual magnetic carrier particles.

The transfer subsystem 50 includes a transfer support member 113 and an intermediate transfer member 112 for transferring respective printed images from the photoreceptor 206 of the imaging member 111 through the first transfer nip 201 to a surface 216 of the intermediate transfer member 112 and thence to the receiver 42, the receiver 42 receiving the respective toned printed images 38 from each printing subsystem in superimposed form to form a composite image thereon. Print image 38 is, for example, a separation of one color (such as cyan). The receiver 42 is transported by a transport web 81. Transfer to the receiver is accomplished by an electric field provided by power supply 240 to transfer support member 113, power supply 240 being controlled by LCU 99. The receiver 42 can be any object or surface to which toner can be transferred from the imaging member 111 by application of an electric field. In this example, receiver 42 is shown prior to entering second transfer nip 202, and receiver 42a is shown subsequent to transfer of printed image 38 onto receiver 42 a.

In the illustrated embodiment, the toner image is transferred from the photoreceptor 206 to the intermediate transfer member 112 and from there to the receiver 42. Registration of the individual toner images is achieved by registering the individual toner images on receiver 42, as is done with NexPress 2100. In some embodiments, a single transfer member is used to sequentially transfer the toner images from each color channel to receiver 42. In other embodiments, individual toner images can be transferred in registration directly from photoreceptors 206 in the

respective printing subsystems

31, 32, 33, 34, 25 to receiver 42 without the use of a transfer member. Any transfer process is suitable in the practice of the present invention. An alternative method of transferring toner images involves transferring individual toner images to a transfer member in registration and then transferring the registered images to a receiver.

LCU 99, among other components, sends control signals to charging subsystem 210, exposure subsystem 220, and respective development stations 225 of each

print subsystem

31, 32, 33, 34, 35 (fig. 1). Each printing subsystem can also have its own respective controller (not shown) coupled to the LCU 99.

Various finishing systems can be used to apply features such as protection, glossing, or binding to the printed image. The finishing system can be implemented as an integral component of printer 100, or can include one or more separate machines through which the printed image is fed after it is printed.

Fig. 3 illustrates a conventional processing path that can be used to produce a printed image 450 using the print engine 370. The preprocessing system 305 is used to process the page-describing file 300 to provide image data 350 in a form ready for printing by the print engine 370. In an exemplary configuration, the pre-processing system 305 includes a Digital Front End (DFE) 310 and an image processing module 330. The pre-processing system 305 can be part of the printer 100 (FIG. 1) or can be a separate system remote from the printer 100. DFE 310 and image processing module 330 can each comprise one or more suitably programmed computers or logic devices adapted to perform operations suitable for providing image data 350.

DFE 310 receives page description file 300 that defines the pages to be printed. The page description file 300 can be in any suitable format (e.g., the well-known Postscript command file format or PDF file format) that specifies the content of pages in terms of text, graphics, and image objects. The image objects are typically provided by an input device such as a scanner, digital camera or computer generated graphics system. The page-describing file 300 can also specify invisible content such as specifications for texture, gloss, or protective coating patterns.

DFE 310 rasterizes page description file 300 into an image bitmap for printing by the print engine. DFE 310 can include various processors, such as a Raster Image Processor (RIP) 315, a color transform processor 320, and a compression processor 325. DFE 310 can also include other processors not shown in fig. 3, such as an image location processor or an image storage processor. In some embodiments, DFE 310 enables a human operator to set parameters such as layout, font, color, media type, or post-trimming options.

RIP 315 rasterizes objects in page description file 300 into an image bitmap comprising an array of image pixels at an image resolution suitable for print engine 370. For text or graphics objects, RIP 315 will create an image bitmap based on the object definition. For image objects, RIP 315 will resample the image data to the desired image resolution.

The color transform processor 320 will transform the image data into the color space required by the print engine 370 to provide color separation for each of the color channels (e.g., CMYK). For the case where the print engine 370 includes one or more additional colors (e.g., red, blue, green, gray, or transparent), the color transform processor 320 will also provide color separation for each of the additional color channels. The objects defined in the page-describing file 300 can be in any suitable input color space, such as sRGB, CIELAB, PCS LAB, or CMYK. In some cases, different color spaces may be used to define different objects. The color transform processor 320 applies the appropriate color transforms to convert the objects into a device-dependent color space for the print engine 370. Methods for creating such color transforms are well known in the color management arts, and any such method can be used in accordance with the present invention. Typically, color transformations are defined using a color management profile that includes a multi-dimensional look-up table. The input color profile is used to define a relationship between an input color space and a Profile Connection Space (PCS) defined for a color management system, e.g., the well-known ICC PCS associated with the ICC color management system. The output color profile defines a relationship between the PCS and a device-dependent output color space for the printer 100. The color conversion processor 320 converts the image data using the color management profile. Typically, the output of the color transform processor 320 will be a color separation set comprising an array of pixels for each of the color channels of the print engine 370 stored in a memory buffer.

The processing applied in digital front end 310 can also include other operations not shown in fig. 3. For example, in some configurations, DFE 310 can apply the halo correction process described in commonly assigned U.S. patent 9147232 (Kuo), entitled "Reducing halo objects in electrophoretic printing systems," which is incorporated herein by reference.

The image data provided by the digital front end 310 is sent to the image processing module 330 for further processing. To reduce the time required to transfer the image data, the compressor processor 325 is typically used to compress the image data using a suitable compression algorithm. In some cases, different compression algorithms can be applied to different portions of the image data. For example, a lossy compression algorithm (e.g., the well-known JPEG algorithm) can be applied to portions of image data that include image objects, and a lossless compression algorithm can be applied to portions of image data that include binary text and graphics objects. The compressed image values are then transmitted over a data link to the image processing module 330, where the compressed image values are decompressed using a decompression processor 335, which applies a corresponding decompression algorithm to the compressed image data by the decompression processor 335.

The halftone processor 340 is configured to apply a halftone process to the image data. Halftone processor 340 can apply any suitable halftone process known in the art. Within the context of the present disclosure, a halftone process is applied to a continuous tone image to provide an image having a halftone dot structure suitable for printing using the printer module 435. The output of the halftone can be a binary image or a multilevel image. In an exemplary configuration, the halftone processor 340 applies the halftone process described in commonly assigned U.S. patent 7830569 (Tai et al) entitled "Multilevel halfone screen and sets thermo of," which is incorporated herein by reference. For this halftoning process, a three-dimensional halftone screen (screen) is provided that includes a plurality of planes that each correspond to one or more intensity levels of the input image data. Each plane defines a pattern of output exposure intensity values corresponding to a desired halftone pattern. The halftone pixel values are multilevel values at a bit depth appropriate for the print engine 370.

The image enhancement processor 345 can apply a variety of image processing operations. For example, the image enhancement processor 345 can be used to apply various image enhancement operations. In some configurations, the image enhancement processor 345 can apply algorithms that modify the halftone process in the border region of the image (see U.S. patent 7079281 entitled "Edge enhancement processor and method with adjustable threshold setting" and U.S. patent 7079287 entitled "Edge enhancement of gray level images" (both to Ng et al), and both of which are incorporated herein by reference).

The preprocessing system 305 provides the image data 350 to the print engine 370, where the image data 350 is printed to provide a printed image 450. The preprocessing system 305 can also provide various signals to the print engine 370 to control the timing at which the image data 350 is printed by the print engine 370. For example, the pre-processing system 305 can signal the print engine 370 to begin printing when a sufficient amount of lines of image data 350 have been processed and buffered to ensure that the pre-processing system 305 will be able to keep up with the rate at which the print engine 370 can print the image data 350.

A data interface 405 in the print engine 370 receives data from the pre-processing system 305. The data interface 405 can use any type of communication protocol known in the art, such as a standard ethernet network connection. The printer module controller 430 controls the printer module 435 according to the received image data 350. In an exemplary configuration, the printer module 435 can be the printer 100 of fig. 1, the printer 100 including a plurality of individual

electrophotographic printing subsystems

31, 32, 33, 34, 35 for each of the color channels. For example, the printer module controller 430 can provide appropriate control signals to activate a light source in the exposure subsystem 220 (FIG. 2) to expose the photoreceptor 206 with an exposure pattern. In some configurations, the printer module controller 430 can apply various image enhancement operations to the image data. For example, the algorithm can be applied to compensate for various sources of non-uniformity in the printer 100 (e.g., streaks formed in the charging subsystem 210, exposure subsystem 220, development station 225, or fuser module 60). One such compensation algorithm is described in commonly assigned U.S. patent 8824907 (Kuo et al) entitled "electrochemical printing with column-dependent turbine adjustment," which is incorporated herein by reference.

In the configuration of fig. 3, the pre-processing system 305 is tightly coupled to the print engine 370 because the pre-processing system 305 must supply the image data 350 in a state that matches the printer resolution and halftone state required for the printer module 435. As a result, when developing new versions of print engines 370 with different printer resolution or halftone status requirements, it has become necessary to also provide updated versions of the pre-processing system 305 that provide the image data 350 in the appropriate state. This has the following disadvantages: requiring the customer to upgrade both the preprocessing system 305 and the print engine 370 at the same time, both the preprocessing system 305 and the print engine 370 may have a significant cost. The present invention addresses this problem by providing an improved print engine design that is compatible with a wide variety of different pre-processing systems.

Fig. 4 shows an improved Print engine 400 as described in commonly assigned U.S. patent 10062017 to c.h. Kuo et al entitled "Print engine with adaptive processing," which is incorporated herein by reference. The improved print engine 400 is adapted to generate a printed image 450 from image data 350, the image data 350 being provided by a plurality of different pre-processing systems 305 configured to supply image data 350 having different image resolutions and halftone states. In an exemplary configuration, the pre-processing system 305 is similar to the system discussed with respect to fig. 3 and includes a digital front end 310 and an image processing module 330. Details of the processing provided by digital front end 310 and image processing module 330 are not included in fig. 4 for clarity, but will operate similarly to the processing discussed with respect to fig. 3. In this case, in addition to supplying the image data 350, the pre-processing system 305 also supplies appropriate metadata 360 that provides an indication of the status of the image data 350. In particular, metadata 360 provides an indication of the image resolution and halftone status of image data 350.

In an exemplary configuration, the metadata 360 includes an image resolution parameter that provides an indication of the image resolution of the image data 350 provided by the pre-processing system 305 and a halftone status parameter that provides an indication of the halftone status of the image data provided by the pre-processing system 305.

The image resolution parameter (R) can take any suitable form that conveys information about the image resolution of the image data 350. In some embodiments, the image resolution parameter can be an integer that specifies the spatial resolution in a suitable unit such as points per inch (dpi) (e.g., R =600 for 600 dpi, and R =1200 for 1200 dpi). In other embodiments, the image resolution parameter can be an index to an enumerated list of allowable spatial resolutions (e.g., R =0 for 600 dpi and R =1 for 1200 dpi).

The halftone state parameter (H) can also take any suitable form. In some embodiments, the halftone state parameter can be a boolean variable that indicates whether applying the halftone process in the pre-processing system 305 causes the image data 350 to be in a halftone state (e.g., H = FALSE indicates that not applying the halftone process causes the image data 350 to be in a continuous tone state, and H = TRUE indicates applying the halftone process causes the image data 350 to be in a halftone state.) in other embodiments, the halftone state parameter can also convey additional information about the type of halftone process that was applied when the halftone process was applied by the pre-processing system 305. For example, the halftone state parameter can be an integer variable, where H =0 indicates that no halftone process has been applied, and other integer values represent an index to the enumerated list of available halftone states (e.g., different screen frequency/angle/dot shape combinations).

The metadata 360 can also specify other relevant pieces of information. For example, for the case where image data 350 is in a continuous tone state such that halftone processor 425 in print engine 400 would be required to apply a halftone operation, metadata 360 can also include one or more halftone parameters used by halftone processor 425 to control the halftone operation. In some embodiments, the halftone parameters can include a screen angle parameter, a screen frequency parameter, or a screen type parameter. In other embodiments, the halftone parameters can include a halftone configuration index for selecting one of a predefined set of halftone algorithm configurations.

Print engine 400 receives image data 350 and metadata 360 using a suitable data interface 405 (e.g., an ethernet interface). The print engine includes a metadata interpreter 410, the metadata interpreter 410 analyzing metadata 360 to provide appropriate control signals 415, the control signals 415 for controlling a resolution modification processor 420 and a halftone processor 425, the resolution modification processor 420 and the halftone processor 425 for processing image data 350 to provide processed image data 428, the processed image data 428 being in an appropriate state to be printed by a printer module 435. The printer module controller 430 then controls the printer module 435 to print the processed image data 428 to produce a printed image 450 in a manner similar to that discussed with respect to fig. 3.

Fig. 5 shows additional details of the resolution modification processor 420 and the halftone processor 425 of fig. 4 according to an exemplary configuration. In this example, control signals 415 provided by metadata interpreter 410 (fig. 4) in response to analyzing metadata 360 (fig. 4) include resolution modification flag 416, resizing factor 417, halftone flag 418, and halftone parameters 419.

The resolution modification flag 416 provides an indication of whether resolution modification must be performed. In an exemplary configuration, the resolution modification flag 416 is a boolean variable that will be set to FALSE if no resolution modification is required (i.e., if the image resolution of the image data 350 matches the printer resolution of the printer module 435) and will be set to TRUE if resolution modification is required.

Halftone flag 418 provides an indication of whether or not a halftone operation is required. In an exemplary configuration, halftone flag 418 is a boolean variable that will be set to FALSE if no halftone operation is required (i.e., if image data 350 is in a halftone state suitable for printer module 435) and will be set to TRUE if a halftone operation must be applied to image data 350 before image data 350 is ready to be printed.

The resolution modification processor 420 applies a modify resolution test 421 to determine if a resolution modification should be performed in response to the resolution modification flag 416. If resolution modification is required, a resolution modification operation 422 is performed. In some configurations, metadata interpreter 410 (fig. 4) provides a resizing factor 417, which resizing factor 417 specifies the amount of resizing that must be provided to adjust the resolution of image data 350 to the resolution required by printer module 435 (fig. 4). In some configurations, the resizing factor 417 is a variable that specifies the ratio between printer resolution and image resolution. For example, if the image data 350 is at 600 dpi, and the printer module 435 is printing at 1200 dpi, the resizing factor 417 would dictate that a 2 × resolution modification is required. In various configurations, the resizing factor 417 can be greater than 1.0 if the printer module 435 has a higher resolution than the image data 350, or the resizing factor 417 can be less than 1.0 if the printer module 435 has a lower resolution than the image data 350.

In an exemplary configuration, if the image resolution of the image data 350 supplied by the pre-processing system 305 is an integer fraction of the printer resolution of the printer module 435, such that the resizing factor 417 is a positive integer, the resolution modification operation 422 performs a resolution modification by performing a pixel copy process. For example, each 600 dpi image pixel in image data 350 would be replaced with a 2 × 2 array of 1200 dpi image pixels each having the same pixel value. In other configurations, an appropriate interpolation process (e.g., nearest neighbor interpolation, bilinear interpolation, or bicubic interpolation) can be used by the resolution modification operation 422. The use of an interpolation algorithm is particularly useful in the case where the resizing factor is not an integer.

For the case where the resizing factor is less than 1.0, the resolution modification operation 422 can perform an appropriate averaging operation to avoid aliasing artifacts. For example, if the resizing factor 417 is 0.5, then 2 x 2 blocks of image pixels in the image data 350 can be averaged together to provide the new resolution. In other configurations, the resolution modification operation 422 can apply a low pass filtering operation followed by a resampling operation.

Halftone processor 425 applies halftone image test 426 to determine whether a halftone operation should be performed in response to halftone flag 418. If a halftone operation is required (e.g., if image data 350 is in a continuous tone state), a halftone operation 427 is performed. In some configurations, metadata interpreter 410 (fig. 4) provides one or more halftone parameters 419 for controlling halftone operations. As previously discussed, halftone parameters 419 can include a screen angle parameter, a screen frequency parameter, or a screen type parameter. In other embodiments, the halftone parameters 419 can include a halftone configuration index for selecting one of a predefined set of halftone algorithm configurations.

The halftoning operations applied by the halftoning processor 425 can use any suitable halftoning algorithm known in the art. In some embodiments, any of the halftone algorithms described in commonly assigned U.S. patent 7218420 (Tai et al) entitled "Gray level halftone processing," commonly assigned U.S. patent 7626730 (Tai et al) entitled "Method of making a Multilevel halftone screen," and commonly assigned U.S. patent 7830569 (Tai et al) entitled "Multilevel halftone screen and sets thermal of," each of which is incorporated herein by reference, can be used. Such halftone algorithms typically involve defining a look-up table that defines halftone dot shapes as a function of position for a square of pixels. Different look-up tables can be defined to produce different halftone dot patterns. For example, different look-up tables can be defined for different screen angles, screen frequencies, and dot shapes. In this case, halftone parameters 419 can include a halftone configuration index that selects which lookup table should be used to halftone image data 350. In a preferred configuration, the halftone processor 425 uses a computational halftone process to compute halftone pixel values using a defined set of operations. An exemplary computational halftone process that can be used in accordance with the present invention is described in the aforementioned U.S. patent 10062017.

Consider the following case: where the printer module 435 prints halftone image data at 1200 dpi, but where a different pre-processing system 305 and configuration can be used to supply image data 350 at 600 dpi or 1200 dpi and in either a halftone state or a continuous tone state. In this case, there will be four different combinations of image resolution parameters and halftone status parameters that the print engine must process.

1. The image resolution parameter indicates that the image data 350 is 600 dpi, and the halftone state parameter indicates that the image data 350 is in a halftone state. In this case, the print engine 400 will print the image data 350 in a mode that simulates a 600 dpi printer. The resolution modification processor 420 will be used to modify the image resolution to provide 1200 dpi data as required by the printer module 435. In an exemplary embodiment, each 600 dpi image pixel is duplicated to provide a 2 × 2 array of 1200 dpi image pixels. Since the image data is already in the halftone state, the halftone operation 427 will be avoided.

2. The image resolution parameter indicates that the image data 350 is 600 dpi, and the halftone state parameter indicates that the image data 350 is in a continuous tone state. In this case, the resolution modification processor 420 would be used to modify the image resolution to provide 1200 dpi data appropriate for the printer module 435, and the halftone processor 425 would apply a halftone operation 427 to the 1200 dpi image data according to the halftone parameters 419.

3. The image resolution parameter indicates that the image data 350 is 1200 dpi, and the halftone state parameter indicates that the image data 350 is in a halftone state. In this case, the image data 350 is already in a state ready for printing by the printer module 435, and thus both the resolution modification operation 422 and the halftone operation 427 will be avoided.

4. The image resolution parameter indicates that the image data 350 is 1200 dpi, and the halftone state parameter indicates that the image data 350 is in a continuous tone state. In this case, because the image data is already at 1200 dpi, such that the resolution modification operation 422 will be bypassed, and the halftone processor 425 will apply the halftone operation 427 to the 1200 dpi image data according to the halftone parameters 419.

As illustrated in fig. 6A, exposure subsystem 220 (fig. 2) in each of

print subsystems

31, 32, 33, 34, 35 (fig. 1) typically includes a print head 475 having a linear array of light sources 460. In the exemplary embodiment, light source 460 is an LED light source, however other types of light sources, such as laser diodes, can also be used. In the illustrated configuration, three light source blocks 470 are used to fabricate the printhead 475, each of the three light source blocks 470 including fifteen light source chips 465. As illustrated in fig. 6B, the light source chip 465 includes a linear array of 384 individual light sources 460. Each of the light sources 460 is connected to a corresponding connection pad 466, and an electrical signal is provided through the connection pad 466 to selectively activate the light sources 460 according to image data. The light sources 460 have a width WS, a height HS, and a light source spacing (i.e., light source-to-light source spacing) PS. In an exemplary configuration, WS =12 μm, HS = 15 μm, and PS = 21.15 μm (corresponding to 1200 dots/inch).

The light source chips 465 are positioned end-to-end in the printhead 475 to form a single array of 384 x 15 x 3 = 17280 light sources 460. Ideally, each of the light sources 460 are spaced apart at exactly the same interval PS so that the light sources 460 expose the photoreceptor 206 in predictable positions. In practice, however, there will be many sources of variability that may introduce cross-track positional errors in the exposed pixels relative to their intended positions. Sources of cross-track positional errors can include variations in light source pitch PS within the light source chip 465, variations in length of the light source chip 465, placement errors in position of the light source chip 465 within the light source block 470, variations in length of the light source block 470, placement errors in position of the light source block 470 within the printhead 475, and placement errors in position of the printhead 475. In addition, light source 460 in printhead 475 is typically imaged onto photoreceptor 206 using a microlens array. The microlenses are typically gradient index "SELFOC" lens rods. Variations in the position and orientation of the microlenses can also introduce variability in the position of the image of the light source 460 on the photoreceptor 206, which will be combined with other sources of variation.

Cross-track positional errors for the light sources 460 may be particularly problematic when the light sources 460 in the print head 475 differ from one

printing subsystem

31, 32, 33, 34, 35 to another, resulting in inter-color registration errors that may be visible and objectionable in many instances. To provide acceptable alignment, the inter-colour alignment error should typically be less than 40 μm, and more preferably should be less than 20 μm. However, under typical manufacturing tolerances, alignment errors of up to 200 μm have been observed. Accordingly, there is a need for a method to characterize and correct cross-track position errors that can be implemented without the need for complex and expensive fixtures.

The title by Kuo et al as incorporated herein by reference is: as described in commonly assigned, co-pending U.S. patent application serial No. 16/417731 to "Correcting cross-track errors in a linear print head," fig. 7 illustrates a flow chart of a method for determining a position correction function 555 that characterizes cross-track position errors associated with a printhead 475 (fig. 6A), according to an exemplary embodiment. The method includes providing digital image data for a test object 500. As illustrated in the exemplary arrangement shown in fig. 8, the test target 500 preferably includes a plurality of alignment marks 570 positioned at predefined cross-track locations. The alignment marks 570 are preferably distributed along the length of the print head 475, with the print head 475 spanning the test target 500 in a cross-track direction 590. The test target 500 may optionally include other content, such as a solid patch 575 that can be used for other calibration or characterization purposes. In an exemplary arrangement, the test target 500 includes alignment marks 570 for a plurality of different color channels. In the illustrated example, the test targets include first color channel image content 580 for a first color channel printed by first printing subsystem 31 (fig. 1), second color channel image content 581 for a second color channel printed by second printing subsystem 32 (fig. 1), third color channel image content 582 for a third color channel printed by third printing subsystem 32 (fig. 1), and fourth color channel image content 583 for a fourth color channel printed by fourth printing subsystem 34 (fig. 1). The image content for each color channel is provided in different image areas distributed along the in-track direction 595. The different color channels can be, for example, black, cyan, magenta, and yellow. However, one skilled in the art will recognize that other colorants can also be used for the color channels. Each of the different image regions includes a corresponding set of alignment marks 570. In other embodiments, rather than using a single test target 500 that includes alignment marks 570 for all color channels, the alignment marks 570 can be included in multiple test targets 500 (e.g., one alignment mark 570 for each color channel).

In the illustrated example of fig. 8, the alignment marks 570 are depicted as an array of equally spaced vertical lines. However, those skilled in the art will recognize that there are a wide variety of different alignment mark spacings and geometries that can be used in accordance with the present invention. In some configurations, the width of the vertical line or cross-track position may vary along the length of the line to enable more accurate measurement of the centroid (centroid) of the printed line. In other cases, the alignment marks can include cross-lines, circles, diamonds, squares, or any other geometric shape that can be analyzed to determine the cross-track position of the alignment marks.

In an exemplary arrangement, the alignment marks 570 are provided proximate to boundaries between adjacent light source chips 465 in the printhead 475 (fig. 6A). This reflects the fact that the most common source of positional error is related to length variability and positioning error for light source chip 465 and light source block 470. Accordingly, forty-four alignment marks 570 would be used for the printhead 475, the printhead 475 including three light source blocks each including fifteen light source chips 465. Preferably, at least ten alignment marks 570 are provided across the length of the printhead 475 to enable characterization and correction of local non-linear cross-track alignment errors.

Returning to the discussion of FIG. 7, a print test object step 505 is used to print test object 500 to produce printed test object 510. In a preferred embodiment, the printed test target 510 is formed on a sheet of receiver 42 (FIG. 2), such as a sheet of paper. In other cases, the printed test target 510 can be an image that is transferred directly onto transport web 81 rather than onto a sheet of receiver 42. In other embodiments, the printed test target 510 can correspond to an intermediate image formed on the surface of the imaging member 111 (i.e., photoreceptor 206) or the surface 216 of the intermediate transfer member 112 (see fig. 2).

The capture image step 515 is followed by capturing a digital image of the printed test target 510 using a digital image capture system to provide a captured image 520. In an exemplary embodiment, the digital image capture system is a flatbed scanner external to the printer 100 for scanning the printed test targets 510 formed on the receiver 42 after the printed test targets 510 have been fully printed and fused. In other embodiments, a digital image capture system (e.g., a digital scanner system or a digital camera system) integrated into printer 100 can be used to capture an image of printed test targets 510 on receiver 42 as receiver 42 is traveling through printer 100 (e.g., as it is being carried on transport web 81), or before it has been transferred to receiver 42 (e.g., on the surface of intermediate transfer member 112 or imaging member 111).

Next, an analyze captured image step 525 is used to automatically analyze the captured image 520 to determine a measured alignment mark position 530. The measured alignment mark positions 530 include at least the cross-track position of the alignment mark 570 in the test target 500. In some embodiments, the measured alignment mark position 530 can also include an in-track position of the alignment mark 570. (the in-track position of the alignment mark 570 can be utilized to correct for artifacts such as substrate skew.) in an exemplary embodiment, a plurality of image rows in the captured image 520 are identified that intersect the alignment mark 570. The image lines are averaged to determine an image trace comprising a combination of traces through the individual alignment marks 570. Likewise, a low pass filter can be applied to the image data to average pixel values over a range of positions within the trajectory, and the combined image trajectory can be determined by taking a single trajectory of the filtered image. Preferably, any skew in the captured image 520 can be characterized (e.g., by detecting the boundaries of the solid patch 575) and interpreted during image analysis. For example, the captured image 520 can be rotated to remove skew. Alternatively, the image trajectory can be taken along a line parallel to the skew angle, or the image can be filtered using a low pass filter rotated at the skew angle.

The combined image tracks can then be analyzed to determine the measured alignment mark positions 530. Fig. 9A shows an example of a combined image trace 526, the combined image trace 526 including an alignment mark contour 527 for each of the alignment marks 570. The "scanner code value" on the y-axis has been inverted so that "0" is white and "255" is black. The measured alignment mark position 530 for each of the alignment marks can then be determined by calculating a quantity corresponding to a measure of the concentration trend for each of the alignment mark profiles 527. For example, the measure of concentration trend can be the center of mass (i.e., average), median, or mode of the alignment mark profile 527.

In an exemplary embodiment, an idealized contour function 528 is fitted to the alignment mark contour 527, as illustrated in fig. 9B. The alignment mark profile 527 in this figure corresponds to the circled alignment mark profile 527 in fig. 9A and has been shifted to remove the density of the paper. The gaussian function was then fitted to the alignment mark profile 527 to determine the idealized profile function 528. The measured alignment mark locations 530 are then determined by calculating a measure of the concentration trend (i.e., centroid) of the idealized contour function 528. This approach has the advantage that it is less susceptible to noise in the image data.

Next, a determine cross track position error step 540 is used to determine a cross track position error 545 by comparing the measured alignment mark position 530 with a corresponding reference alignment mark position 535. In some embodiments, the reference alignment mark location 535 can correspond to an ideal location of the alignment mark 570 determined from the location of the alignment mark 570 in the original test target 500. In a preferred embodiment, one of the color channels is designated as a reference color channel and the other color channels are designated as non-reference color channels. In this case, the measured alignment mark position 530 for the reference color channel is used as the reference alignment mark position 535 for the non-reference color channel. In this way, the cross-track position error 545 for the non-reference color channel corresponds to the cross-track difference between the image content printed in the non-reference color channel and the reference color channel. In some configurations, a predefined color channel (e.g., a black color channel) is designated as the reference color channel. In other cases, it can be advantageous to designate the color channel having the largest cross-track line length (e.g., the color channel having the largest cross-track distance between the first alignment mark and the last alignment mark) as the reference color channel. In this case, the position correction applied to the non-reference color channels will stretch the image data (e.g., by repeating certain image pixels) rather than shorten the image data (e.g., by deleting certain image pixels). This excludes the possibility that a portion of a single pixel wide line can be erased by deleting the corresponding image pixel.

Fig. 10A illustrates a cross-track position error 545 determined for a printed test target 510 generated using an exemplary printhead 475. The cross-track position error 545 was determined by calculating the difference between the measured alignment mark position 530 and the corresponding reference alignment mark position 535. The positive cross-track position error 545 corresponds to a case where the position of the alignment mark in the printed image is longer than the reference position (i.e., to the right), and the negative cross-track position error 545 corresponds to a case where the printed image is shorter than the reference position (i.e., to the left). It can be seen in this example that one portion of the print head has a negative cross-track position error, while another portion of the print head has a positive cross-track position error, which indicates that the spacing between the light sources varies across the width of the print head.

Determine position correction function step 550 is then used to determine a position correction function 555 based on the measured cross-track position error 545. The position error in this example is scaled by the output pixel spacing such that the position error is expressed in terms of a number of output pixels (e.g., a number of 1200 dpi pixels). In an exemplary embodiment, a smoothing function is fitted to the measured cross-track position error 545 to determine a cross-track position error function 546. For example, the cross-track position error function 546 can be determined by fitting a smooth spline or polynomial function to the measured cross-track position error 545. Such smoothing operations are well known to those skilled in the art.

In an exemplary embodiment, the correction is applied by resampling the image data. In this case, the resampling operation effectively shifts the image data by an integer number of output pixels as a function of pixel position. The required offset can be determined by quantizing the cross-track position error function 546 to determine a quantized cross-track position error function 547. The quantized cross-track position error function 547 gives an indication of how many pixels the output pixel position has been shifted to the right or left. For example, the quantized position error for pixel indices within the range of 1357-.

To correct for the cross-track position error, the position correction function 555 can be determined by inverting the quantized cross-track position error function 547, as shown in fig. 10B. In an exemplary embodiment, the correction is applied by resampling the image data at the shifted pixel locations. The position correction function 555 gives an indication of how many output pixels the image data should be shifted as a function of cross-track pixel position.

The representation of the position correction function 555 can be stored in digital memory in any suitable format to be used in the correction of digital image data. For example, the full position correction function 555 can be stored in digital memory in quantized form such as illustrated in fig. 10B or in unquantized form. Alternatively, the location correction function 555 can be represented in other formats. For example, the quantized position correction function 555 of fig. 10B can be comprehensively represented by storing the differences between the quantized position correction values at sequential pixel positions. An example of such a location correction function representation 560 is illustrated in fig. 10C. The location correction function representation 560 can be stored in digital memory in a variety of encoding formats. For example, the difference (i.e., which can also be referred to as a "transition direction" or "delta modulation value") can be stored as a function of the pixel index. Alternatively, the cross-track position and transition direction (i.e., delta modulation value) of the transition (i.e., pixel index with non-zero delta modulation value) where the quantized position correction value changes can be stored in a table such as that shown in table 1.

TABLE 1 Cross-track position correction function representation

Pixel index	Delta modulation value
		1357	+1
6442	-1
		10204	-1
11957	-1
		13318	-1
15042	-1

Once the location correction function 555 has been determined, the image lines of the digital image can be modified in response to the stored location correction function to determine corrected image lines. In a preferred embodiment, the image line is resampled at locations corresponding to pixel offsets specified in a position correction function 555 such as that shown in FIG. 10C.

FIG. 11 illustrates an improved processing path including a print engine adapted to generate a printed image incorporating cross-track position correction according to an exemplary embodiment. The improved processing path is similar to that of fig. 4, except that: the resolution modification processor 420 has been replaced by a resolution/alignment processor 600, the resolution/alignment processor 600 correcting the alignment in response to a position correction function 555 in addition to performing any resolution modifications specified by the control signal 415.

Fig. 12 shows additional detail for the resolution/alignment processor 600 and the halftone processor 425 of fig. 11. The process is similar to that of fig. 5, except that a position correction operation 610 is added. As previously discussed, the resolution modification operation 422 involves resampling the image data 350 according to a resizing factor. The position correction operation 610 also involves resampling of the image data. In an exemplary embodiment, the resolution modification operation 422 and the position correction operation 610 can be combined into a single unified resampling operation 620 rather than two sequential resampling operations.

In an exemplary embodiment, unified resampling operation 620 uses a "nearest neighbor" resampling process, wherein each output pixel is set to the value of the input pixel closest to the corresponding sampling location. This ensures that the density of thin lines and text is maintained. In other embodiments, an interpolation process can be used to interpolate between input pixel values to determine an output pixel value at the determined sampling location.

Fig. 13 illustrates an exemplary method for processing input pixels 630 of image data 350 (fig. 12) having an associated cross-track pixel index 635 using unified resampling operation 620 of fig. 12. This exemplary method corresponds to a special case where the resizing factor 417 is 2 × (e.g., when image data 350 (fig. 12) has a resolution of 600 dpi and processed image data 428 (fig. 12) has a resolution of 1200 dpi). The determine delta modulation value step 640 is for determining a delta modulation value corresponding to the pixel index 635 in response to the pixel correction function 555 (a)

)645. For example, pixel index 635 can be used for a pixel such as that shown in FIG. 10BDelta modulation values 645 are found in the sample position correction function 555. Alternatively, the pixel index 635 can be compared to pixel indices in a table such as that shown in table 1 to determine whether the delta modulation value 645 is non-zero, and if so, what its value should be.

The adder 650 is then used to combine the size-reset factor 417 and the delta-modulation value 645 to determine the repetition value 670. The repeat value 670 indicates how many times the input pixel 630 should be repeated in a row of output pixels 680. For example, if the size-resetting factor 417 is 2 x, and the delta modulation value 645 is

=0, the repeat value 670 will have a nominal value of "2" such that the input pixel 630 will repeat twice according to the size-reset factor 417. If delta modulation value 645 is

= -1 or

= +1, the repeat value 670 will be adjusted to "1" or "3", respectively, to correct the cross track position error.

Repeat input pixels step 675 is then used to determine an output pixel 680 corresponding to input pixel 630 by repeating input pixel 630 a number of times (e.g., 1, 2, or 3 times) according to repeat value 670. The process of FIG. 13 is repeated for each input pixel 630 in each image line of image data 350 (FIG. 12). Note that each determined row of output pixels 680 will be repeated twice in the output image data, taking into account the 2 x resizing factor 417.

For the case where the size-resetting factor 417 is 1 x,

a delta modulation value 645 of =1 will give a repetition value 670 of "0". The consequence of this would be that if the input pixel 630 corresponded to a single pixel wide line, the input pixel 630 would be from the output mapLike erasing. To avoid such artifacts, it is generally desirable to avoid the negative delta modulation value 645 if the size-resetting factor is 1 ×. This can generally be done by designating the color channel determined to have the longest cross-track line length as the reference color channel. In this way, the lengths of the other color channels will be stretched rather than compressed.

Even if the resizing factor 417 is 2 x or greater, the non-zero delta modulation value 645 can cause the line width of a thin line (e.g., a single pixel wide line) to be modified to the extent that a user can detect the difference. For example, a line that would normally be two output pixels wide after applying the 2 × resizing factor 417 might be one or three output pixels wide. To avoid such artifacts, it is generally desirable to avoid aligning the non-zero delta modulation values 645 with fine features in the input image. In one embodiment, a plurality of different position correction functions 555 can be provided in which the cross-track position of the transition is shifted to the left or right. If the user observes an objectionable change in feature width, the user can select one of the alternative location correction functions 555. In other embodiments, the input image can be analyzed to identify the location of the fine image feature, and the location of the transition can be offset such that the transition moves away from the fine image feature (e.g., into a white background region).

In some embodiments, as previously discussed, printer 100 (fig. 1) includes an image capture system that can be used to capture images of printed test targets 510 on a suitable imaging surface. In such a case, the calibration method of fig. 7 can be performed automatically without requiring the user to manually handle the printed test target 510. The calibration method can be performed at predefined intervals or can be initiated by the user upon observing that the printer is producing a printed image with objectionable cross-track position errors.

The methods for correcting cross-track alignment errors described with respect to fig. 7-13 can be adapted to also correct in-track alignment errors. FIG. 14 illustrates a flow diagram of a method for determining an in-track position correction function 855 that characterizes and corrects an in-track position error associated with a printhead 475 (FIG. 6A), according to an exemplary embodiment. The in-track positional error may result from a variety of sources, including skew of the printhead 475 relative to the imaging member 111 (fig. 2), misalignment of individual light source chips 465 or light source blocks 470 within the printhead 475, misalignment of imaging optics (e.g., a SELFOC lens), or distortion of the imaging member 111. The method includes providing digital image data for a test object 800. As illustrated in the exemplary arrangement shown in fig. 15, the test target 800 preferably includes a plurality of in-track alignment marks 870 positioned at predefined cross-track locations. In-track alignment marks 870 are preferably distributed along the length of printhead 475, with printhead 475 spanning test target 800 in a cross-track direction 590. The test target 800 may optionally include other content, such as cross-track alignment marks 570 that can be used to correct cross-track alignment errors and solid patches 575 that can be used for other calibration or characterization purposes, as has been previously described. In the exemplary arrangement, the test target 800 includes in-track alignment marks 870 for a plurality of different color channels. In the illustrated example, the test targets include first color channel image content 580 for a first color channel printed by first printing subsystem 31 (fig. 1), second color channel image content 581 for a second color channel printed by second printing subsystem 32 (fig. 1), third color channel image content 582 for a third color channel printed by third printing subsystem 32 (fig. 1), and fourth color channel image content 583 for a fourth color channel printed by fourth printing subsystem 34 (fig. 1). Each of the different color channels includes a corresponding set of in-track alignment marks 870. The different color channels can be, for example, black, cyan, magenta, and yellow. However, one skilled in the art will recognize that other colorants can be used for the color channels. In other embodiments, rather than using a single test target 800 that includes in-track alignment marks 870 for all color channels, in-track alignment marks 870 can be included in multiple test targets 800 (e.g., there is one in-track alignment mark 870 for each color channel).

In the illustrated example of fig. 15, the in-track alignment marks 870 are depicted as an array of equally spaced horizontal lines that are all positioned at the same nominal position along the in-track direction 595. However, those skilled in the art will recognize that there are a wide variety of different alignment mark spacings and geometries that can be used in accordance with the present invention. In some configurations, the width of the horizontal line or the in-track position may vary along the length of the line to enable more accurate measurement of the centroid of the printed line. In other cases, the in-track alignment marks 870 can include cross-lines, circles, diamonds, squares, or any other geometric shape that can be analyzed to determine the in-track position of the alignment marks. In some cases, the cross-track alignment mark 570 and the in-track alignment mark 870 can be combined into an alignment mark adapted to enable determination of a single set of both the in-track and cross-track positions of the alignment mark.

In an exemplary arrangement, the in-track alignment marks 870 are provided proximate to boundaries between adjacent light source chips 465 in the printhead 475 (fig. 6A). This reflects the fact that some of the most common sources of in-track positional errors are related to positioning errors for light source chip 465 and light source block 470. Accordingly, forty-four in-track alignment marks 870 can be used for the printhead 475, the printhead 475 including three light source dice that each include fifteen light source chips 465. In other arrangements, multiple sets of in-track alignment marks 870 can be provided for each light source chip 465. For example, two sets of in-track alignment marks 870 can be provided for each light source chip 465, one closer to the left side edge and one closer to the right side edge. Preferably, at least ten in-track alignment marks 870 are provided across the length of the printhead 475 to enable characterization and correction of local non-linear cross-track alignment errors.

Returning to the discussion of FIG. 14, print test object step 805 is used to print test object 800 to produce printed test object 810. In a preferred embodiment, the printed test object 810 is formed on a sheet of receiver 42 (FIG. 2), such as a sheet of paper. In other cases, the printed test object 810 can be an image that is transferred directly onto transport web 81 rather than onto a sheet of receiver 42. In other embodiments, the printed test targets 810 can correspond to intermediate images formed on the surface of the imaging member 111 (i.e., photoreceptor 206) or the surface 216 of the intermediate transfer member 112 (see fig. 2).

The capture image step 815 is followed by capturing a digital image of the printed test target 810 using a digital image capture system to provide a captured image 820. In an exemplary embodiment, the digital image capture system is a flatbed scanner external to printer 100 for scanning printed test objects 810 formed on receiver 42 after printed test objects 810 have been completely printed and fused. In other embodiments, a digital image capture system (e.g., a digital scanner system or a digital camera system) integrated into printer 100 can be used to capture an image of printed test targets 810 on receiver 42 while receiver 42 is traveling through printer 100 (e.g., while it is being carried on transport web 81), or before it has been transferred to receiver 42 (e.g., on the surface of intermediate transfer member 112 or imaging member 111).

Next, an analyze captured image step 825 is used to automatically analyze the captured image 820 to determine a measured in-track alignment mark position 830. The measured in-track alignment mark positions 830 include at least the in-track positions of in-track alignment marks 870 (FIG. 15) in the test target 800. In an exemplary embodiment, a plurality of image columns in the captured image 820 that intersect the in-track alignment marks 870 are identified. The image columns are averaged to determine a combined image track (which can also be referred to as an in-track alignment mark outline) for each of the individual in-track alignment marks 870. Likewise, a low pass filter can be applied to the image data to average pixel values over a range of cross-track positions, and the combined image trajectory can be determined by taking a single trajectory of the filtered image. Preferably, any skew in the captured image 820 can be characterized (e.g., by detecting the boundaries of the solid patch 575) and interpreted during image analysis. For example, the captured image 820 can be rotated to remove skew. Alternatively, the image trajectory can be taken along a line parallel to the skew angle, or the image can be filtered using a low pass filter rotated at the skew angle. The in-track alignment mark profile is then analyzed to determine the measured alignment mark position 830. In an exemplary embodiment, the idealized profile function 528 fits to the in-track alignment mark profile in a similar manner as previously described in the discussion of fig. 9B with respect to the cross-track alignment mark profile 527. The measured in-track alignment mark position 830 is then determined by calculating a measure of the concentration trend (i.e., the centroid) of the idealized contour function 528. This approach has the advantage that it is less susceptible to noise in the image data.

Next, an in-track position error determining step 840 is used to determine an in-track position error 845 by comparing the measured in-track alignment mark position 830 with the corresponding reference in-track alignment mark position 835. In some embodiments, the alignment mark positions 835 within the reference track can correspond to ideal positions of the alignment marks 870 corresponding to the positions of the alignment marks 870 in the original test target 800. In some embodiments, the reference in-track alignment mark position 835 can correspond to the measured in-track alignment mark position 830 for one of the alignment marks (e.g., the leftmost alignment mark or the center alignment mark). In some embodiments, one of the color channels is designated as a reference color channel and the other color channels are designated as non-reference color channels. In this case, the intra-reference track alignment mark position 835 for the non-reference color channel can be specified, taking into account the known relative positions of the alignment mark positions in the original test target 800. In this way, in addition to any in-channel skew, the in-track position error 845 for the non-reference color channel will also reflect any inter-channel registration error.

Fig. 16A illustrates an in-track position error 845 determined for a printed test target 810 produced using an exemplary print head 475. The in-track position error 845 was determined by calculating the difference between the measured in-track alignment mark position 830 and the corresponding reference in-track alignment mark position 835. The positive cross-track position error 845 corresponds to a case where the position of the in-track alignment mark in the printed image is higher than the reference position on the printed test target 810 (i.e., assuming that the top of the image is printed first, downstream with respect to the printing direction), and the negative in-track position error 845 corresponds to a case where the printed image is lower than the reference position (i.e., assuming that the top of the image is printed first, upstream with respect to the printing direction). In this example, the printhead 475 is skewed such that the right edge of the printed image is printed higher on the page than the left edge. In addition, there are some local deviations in the position within the track.

The determine in-track position correction function step 850 is then used to determine an in-track position correction function 855 based on the measured in-track position error 845. The in-track position error in this example is scaled by the output pixel spacing such that the in-track position error is expressed in terms of a number of output pixels (e.g., a number of 1200 dpi pixels). In an exemplary embodiment, a smoothing function is fitted to the measured in-track position error 845 to determine an in-track position error function 846. For example, the in-track position error function 846 can be determined by fitting a smooth spline or polynomial function to the measured in-track position error 845. Such smoothing operations are well known to those skilled in the art.

In an exemplary embodiment, the in-orbit alignment correction is applied by resampling the image data. In this case, the resampling operation effectively shifts the image data by an integer number of output pixels in the intra-track direction as a function of the cross-track pixel location. The required offset can be determined by quantizing the in-track position error function 846 to determine a quantized in-track position error function 847. The quantized in-track position error function 847 gives an indication of how many pixels the output pixel position has been shifted up or down. For example, the quantized in-track position error for the cross-track pixel index in the range of 645-2965 indicates that the pixel is about one pixel lower than its expected position.

To correct for in-track position errors, an in-track position correction function 855 can be determined by inverting the quantized in-track position error function 847, as shown in fig. 16B. In an exemplary embodiment, the correction is applied by resampling the image data at the shifted pixel locations. The in-track location correction function 855 gives an indication of how many output pixels the image data should be shifted in the in-track direction as a function of cross-track pixel location.

The representation of the in-track position correction function 855 can be stored in digital memory in any suitable format to be used in the correction of digital image data. For example, the full intra-track position correction function 855 can be stored in digital memory in quantized form such as illustrated in fig. 16B or in unquantized form. Alternatively, the in-track location correction function 855 can be represented in other formats. For example, the quantized in-track position correction function 855 of fig. 16B can be fully represented by storing the differences between the quantized position correction values at sequential pixel positions. An example of such an in-track position correction function representation 860 is illustrated in fig. 16C. The in-track position correction function representation 860 can be stored in digital memory in a variety of encoding formats. For example, the difference (i.e., which can also be referred to as a "transition direction" or "delta modulation value") can be stored as a function of the pixel index. Alternatively, the cross-track position and transition direction (i.e., delta modulation value) of the transition (i.e., pixel index with non-zero delta modulation value) where the quantized position correction value changes can be stored in a table such as that shown in table 2.

Table 2. in-orbit position correction function representation.

Pixel index	Delta modulation value
		645	1
2966	-1
		7458	-1
7953	1
		8995	-1
10971	-1
		11396	1
12577	-1
		16854	-1
17172	-1

Once the in-orbit position correction function 855 has been determined, the image lines of the digital image can be modified in response to the stored in-orbit position correction function to determine corrected image lines. In a preferred embodiment, the image rows are offset in an in-track direction, wherein the offset varies as a function of the cross-track position according to an in-track position correction function 855.

FIG. 17 illustrates an improved processing path including a print engine adapted to generate a printed image incorporating cross-track position correction according to an exemplary embodiment. The improved processing path is similar to that of fig. 11, except that: resolution/alignment processor 600 has been replaced by a new resolution/alignment processor 900, and in addition to performing any resolution modifications specified by control signal 415, resolution/alignment processor 900 also corrects for alignment in response to both cross-track position correction function 555 and in-track position correction function 855.

Fig. 18 shows additional detail for the resolution/alignment processor 900 and the halftone processor 425 of fig. 17. The process is similar to that of fig. 12 except for the location correction operation 910, which location correction operation 910 applies both a cross-track location correction function 555 and an in-track location correction function 855. As previously discussed, the resolution modification operation 422 involves resampling the image data 350 according to a resizing factor. The position correction operation 910 also involves resampling of the image data. In an exemplary embodiment, the resolution modification operation 422 and the location correction operation 910 can be combined into a single unified resampling operation 920 instead of two sequential resampling operations.

In an exemplary embodiment, the unified resampling operation 920 works by first performing cross-track resizing and position correction operations using the process previously described with respect to fig. 13. An in-track resizing operation is then performed by copying the processed lines to provide buffered image lines at the output resolution. An in-track position correction operation is then performed in which the output image line is determined by resampling the buffered image lines according to an in-track position correction function 855, preferably expressed in terms of the number of output pixels for which the image data should be shifted as a function of cross-track position.

An exemplary embodiment of an in-track position correction operation is illustrated in fig. 19, where fig. 19 shows an image buffer 930 containing nine image lines, where the center image line corresponds to the nominal image for a particular in-track position y 0. (Note that for purposes of illustration, the image lines in this example are shortened relative to the real image lines, which can have up to 17000 pixels or more.) for each cross-track pixel index i, the image buffer 930 is sampled at an offset in-track position yi given by:

where cy (i) is the value of the in-track location correction function 855 evaluated at the ith pixel index. The shaded pixel locations in image buffer 930 indicate selected pixel locations corresponding to an exemplary in-track location correction function 855. The pixel values at these pixel locations are copied into output image row 940. For the case where a delta modulation function is used as the in-track location correction function representation 860, the offset in-track location yi for each cross-track pixel index can be determined by incrementing the offset in-track location for the previous cross-track pixel index by the in-track delta modulation value for that cross-track pixel index,

wherein the content of the first and second substances,

is the in-track delta modulation value for the ith cross-track pixel index.

The image buffer 930 should include at least as many image lines as are needed to cover the expected range of maximum values of correction for the in-orbit position correction function 855. After each output image line 940 is processed, the image lines in the image buffer 930 are shifted upwards and a new image line is added to the bottom of the image buffer 930. In an alternative embodiment, image buffer 930 is capable of storing the entire image. This results in no need to perform image line shifting operations, but requires a much larger amount of memory, which may be impractical in many systems.

For the case where an intra-row size reset factor of 2 x or greater is used, there will be redundant image rows in image 930, which is an inefficient use of buffer memory. In such cases, it can be advantageous to integrate the in-track resizing operation with the in-track position correction operation. In an exemplary embodiment, an image buffer can be used to store image lines prior to performing an in-track resizing operation. The image line index for each pixel location yi can be previously determined to correspond to the output image resolution and can be mapped to a corresponding image line in an image buffer containing pre-in-track resized image lines

：

Where M is a size-resetting factor 417, and,

is a function that returns the integer part of the number. (Note that the same resizing factor will typically be used in both the in-track and cross-track directions, however this is not a requirement.)

As previously discussed, in some embodiments, the in-track location correction function 855 for each color channel can be determined with respect to the reference color channel such that the in-track location correction function 855 will not only correct the skew of the individual color channels, but will also account for inter-color registration errors. In other cases, the overall inter-color registration error can be performed separately, for example, by introducing a time delay into the printing operation for the non-reference color channels that corresponds to the overall shift detected between the color channels.

In some embodiments, as previously discussed, printer 100 (fig. 1) includes an image capture system that can be used to capture images of printed test targets 810 on a suitable imaging surface. In such a case, the calibration method of fig. 14 can be performed automatically without requiring the user to manually handle the printed test targets 810. The calibration method can be performed at predefined intervals or can be initiated by the user upon observing that the printer is producing a printed image with objectionable in-track position errors.

FIG. 20 is a high-level diagram illustrating components of a system for processing image data according to an embodiment of the present invention. The system includes a data processing system 710, a peripheral system 720, a user interface system 730, and a data storage system 740. Peripheral system 720, user interface system 730, and data storage system 740 are communicatively connected to data processing system 710.

Data processing system 710 includes one or more data processing devices that implement the processes of various embodiments of the present invention, including the exemplary processes described herein. The phrase "data processing device" or "data processor" is intended to include any data processing device, such as a central processing unit ("CPU"), desktop computer, laptop computer, mainframe computer, personal digital assistant, blackberry, digital camera, cellular telephone, or any other device that: whether implemented with electrical, magnetic, optical, biological, or otherwise, components for processing, managing, or handling data. In some embodiments, the data processing system 710 is a plurality of data processing devices distributed throughout various components of the printing system (e.g., the pre-processing system 305 and the print engine 370).

Data storage system 740 includes one or more processor-accessible digital memories configured to store information, including information needed to perform the processes of various embodiments of the present invention, including the example processes described herein. Data storage system 740 may be a distributed processor-accessible memory system that includes a plurality of processor-accessible digital memories communicatively connected to data processing system 710 via a plurality of computers or devices. On the other hand, data storage system 740 need not be a distributed processor-accessible digital memory system, and thus may include one or more processor-accessible digital memories located within a single data processor or device.

The phrase "processor-accessible digital memory" is intended to include any processor-accessible data storage device, whether volatile or non-volatile, electronic, magnetic, optical, or otherwise, including, but not limited to, registers, floppy disks, hard disks, compact disks, DVDs, flash memory, ROM, and RAM.

The phrase "communicatively connected" is intended to include any type of connection, whether wired or wireless, between devices, data processors, or programs in which data may be communicated. The phrase "communicatively connected" is intended to include a connection between devices or programs within a single data processor, a connection between devices or programs located in different data processors, and a connection between devices not located in a data processor at all. In this regard, although data storage system 740 is shown separately from data processing system 710, one skilled in the art will also appreciate that data storage system 740 may be stored wholly or partially within data processing system 710. Also in this regard, although the peripheral system 720 and the user interface system 730 are shown separately from the data processing system 710, one skilled in the art will also appreciate that one or both of such systems may be stored wholly or partially within the data processing system 710.

Peripheral system 720 may include one or more devices configured to provide digital content records to data processing system 710. For example, peripheral system 720 may include a digital camera, digital video camera, cellular telephone, or other data processor. When receiving digital content records from devices in peripheral system 720, data processing system 710 may store such digital content records in data storage system 740.

The user interface system 730 may include a mouse, a keyboard, another computer, or any device or combination of devices from which data is input to the data processing system 710. In this regard, although the peripheral system 720 is shown separately from the user interface system 730, the peripheral system 720 may be included as part of the user interface system 730.

The user interface system 730 may also include a display device, a processor-accessible memory, or any device or combination of devices to which data is output by the data processing system 710. In this regard, if the user interface system 730 includes a processor-accessible memory, such memory may be part of the data storage system 740 even though the user interface system 730 and the data storage system 740 are shown separately in FIG. 20.

A computer program product for performing aspects of the invention can include one or more non-transitory tangible computer-readable storage media, e.g., a computer readable storage medium; magnetic storage media such as a magnetic disk (such as a floppy disk) or magnetic tape; optical storage media such as optical disks, optical tape, or machine-readable bar codes; solid state electronic storage devices, such as Random Access Memory (RAM) or Read Only Memory (ROM); or any other physical device or medium employed to store a computer program having instructions for controlling one or more computers to practice the method according to the present invention.

The inventive method for correcting cross-track and in-track position errors has been described within the context of an electrophotographic printer 100 (fig. 1) that utilizes a linear printhead having a linear array of light sources for exposing a photoreceptor 206 (fig. 2). It will be apparent to those skilled in the art that the method can be equally used to correct cross-track and in-track positional errors in other types of digital printers that include linear arrays of light sources, such as printers for writing on other types of photosensitive media (e.g., printers for exposing silver halide photographic paper). The method can similarly be used to correct in-track and cross-track positional errors associated with other types of linear printheads, such as inkjet printheads that include a linear array of ejection nozzles for ejecting ink drops onto a receiver medium.

Parts list

31 printing subsystem

32 print subsystem

33 print subsystem

34 print subsystem

35 printing subsystem

38 printing an image

39 fused image

40 supply unit

42 receiver

42a receiver

42b receiver

50 transfer subsystem

60 fuser module

62 fusion roller

64 pressure roller

66 fusion nip

68 Release fluid application substation

69 output tray

70 finishing machine

81 conveying net

86 cleaning station

99 Logic and Control Unit (LCU)

100 Printer

111 imaging member

112 intermediate transfer member

113 transfer support member

201 first transfer nip

202 second transfer nip

206 light sensor

210 charging subsystem

211 instrument

212 meter

213 grid

216 surface

220 exposure subsystem

225 developing station

226 toning shell

227 magnetic core

240 power supply

300 page description file

305 preprocessing system

310 Digital Front End (DFE)

315 Raster Image Processor (RIP)

320 color transform processor

325 compression processor

330 image processing module

335 decompression processor

340 halftone processor

345 image enhancement processor

350 image data

360 metadata

370 print engine

400 print engine

405 data interface

410 metadata interpreter

415 control signal

416 resolution modification flag

417 size-resetting factor

418 halftone mark

419 halftone parameter

420 resolution modification processor

421 modified resolution test

422 resolution modification operation

425 halftone processor

426 halftone image test

427 halftone operation

428 processed image data

430 printer module controller

435 printer module

450 printed image

460 light source

465 light source chip

466 connection pad

470 light source block

475 print head

500 test object

505 print test target step

510 printed test target

515 image capturing step

520 captured image

525 analyzing the captured image step

526 Combined image track

527 alignment mark profile

528 idealized profile function

530 measured alignment mark position

535 reference alignment mark position

540 step of determining cross-track position error

545 cross-track position error

546 cross-track position error function

547 quantized Cross-track position error function

550 determining a location correction function step

555 position correction function

560 location correction function representation

570 alignment mark

575 solid patch

580 first color channel image content

581 second color channel image content

582 third color channel image content

583 fourth color channel image content

590 cross-track direction

595 in-orbit direction

600 resolution/alignment processor

610 position correction operation

620 unified resampling operation

630 input pixel

635 Pixel index

640 determine a delta modulation step

645 delta modulation value

650 adder

670 repetition value

675 repeat input pixel step

680 output pixel

710 data processing system

720 peripheral system

730 user interface system

740 data storage system

800 test object

805 print test target step

810 printed test target

815 step of capturing an image

820 captured image

825 analyzing the captured image step

830 measured position of alignment mark in track

835 reference track inner alignment mark position

840 determining in-track position error

845 in-orbit position error

846 in orbit position error function

847 quantized in-track position error function

850 determining an in-track position correction function step

855 in-orbit position correction function

860 in-orbit position correction function representation

870 in-orbit alignment marks

900 resolution/alignment processor

910 position correction operations

920 unified resampling operation

930 image buffer

940 outputs the image lines.

Claims

1. A method for correcting in-track position errors in a digital printing system having a linear printhead extending in a cross-track direction and including an array of light sources for exposing a photosensitive medium, the method comprising:

e) comparing the measured in-track position for the alignment mark to a reference in-track position to determine a measured in-track position error;

f) determining an in-track position correction function in response to the measured in-track position error, wherein the in-track position correction function specifies an in-track position correction to be applied as a function of cross-track position;

h) receiving digital image data for a digital image to be printed by the digital imaging system, wherein the digital image includes a plurality of image lines extending in the cross-track direction;

i) determining a corrected image line by resampling the digital image data in response to the stored representation of the in-orbit position correction function; and

j) printing the corrected image lines using the digital printing system to provide a printed image with reduced in-track position errors.

2. The method of claim 1, wherein the digital printing system includes a plurality of color channels, wherein one of the color channels is designated a reference color channel and the other color channels are designated non-reference color channels, and wherein the reference position for the alignment mark for the non-reference color channel is determined in response to the measured position of one or more alignment marks printed with the reference color channel.

3. The method of claim 2, wherein a predefined color channel is designated as the reference color channel.

4. The method of claim 1, wherein the reference position corresponds to an ideal position of the alignment mark.

5. The method of claim 1, wherein the printed test targets are on a print medium, and wherein the digital image capture system captures images of the printed test targets on the print medium.

6. A digital printing system incorporating in-orbit position correction, comprising:

one or more printing subsystems, each printing subsystem comprising a linear printhead extending in a cross-track direction, the linear printhead comprising an array of light sources for exposing a photosensitive medium;

a data processing system;

a digital memory for storing an in-track position correction function; and

a program memory communicatively connected to the data processing system and storing instructions configured to cause the data processing system to implement a method for determining an in-track position correction function for at least one printing subsystem, wherein the method comprises:

d) automatically analyzing the captured images to determine a measured in-track position for each of the alignment marks;

f) determining the in-track position correction function in response to the measured in-track position error, wherein the in-track position correction function specifies an in-track position correction to be applied as a function of cross-track position; and

g) storing a representation of the in-track position correction function in the digital memory;

wherein the digital printing system is adapted to print a digital image using a printing process comprising:

i) receiving digital image data for a digital image to be printed by the digital imaging system, wherein the digital image includes a plurality of image lines extending in the cross-track direction;

ii) determining a corrected image line by resampling the digital image data in response to the stored representation of the in-orbit position correction function; and

iii) printing the corrected image lines using the one or more printing subsystems to provide a printed image with reduced in-track position errors.

7. The digital printing system of claim 6, wherein the digital printing system includes a plurality of printing subsystems for printing a corresponding plurality of color channels, wherein one of the color channels is designated a reference color channel and the other color channels are designated non-reference color channels, and wherein the reference position for the alignment mark for the non-reference color channel is determined in response to the measured position of one or more alignment marks printed with the reference color channel.

8. The digital printing system of claim 7, wherein a predefined color channel is designated as the reference color channel.

9. The digital printing system of claim 6, wherein the reference position corresponds to an ideal position of the alignment mark.

10. The digital printing system of claim 6, wherein the printed test targets are on a print medium, and wherein the digital image capture system captures images of the printed test targets on the print medium.

11. The digital printing system of claim 6, wherein the printed test targets are on an imaging surface that is a surface of a photoconductor, a surface of an intermediate transfer member, or a surface of a transport web, and wherein the digital image capture system captures images of the printed test targets on the imaging surface.